France enters the Matrix

However, the novelty here is that unlike HTTP or SMTP, the scanning is done in a manner that works in an E2E-encrypted-by-default network. I guess the equivalent for email would be:

* Have your MTA strip off attachments and store them in IMAP or some other content repository
* Refuse to let MUAs access those attachments other than by proxying them through an AV scanner
* If the attachment is PGP encrypted, then the MUA has to hand the scanner the encrypted keys to that particular attachment so that the scanner can do its job.
* The scanner runs entirely operationally isolated from the MTA.

As far as I know, nobody has ever built such a system for email, probably because E2E encryption is far from commonplace on email, and doesn't lend itself to scanning specific attachments (given PGP acts on the whole envelope, rather than each attachment being separately encrypted).

It's worth noting that whilst we're going to spec the API for doing this dance (i.e. formalise the stuff in https://github.com/matrix-org/matrix-content-scanner#api and add it to the matrix spec), this API will of course categorically *not* be compsulsory. It's just providing a hook for content scanning for those users (e.g. France) for whom it's a requirement, such that Matrix clients can perform such scanning in a consistent way.

It's an interesting debate as to whether this should be a network service in the first place, or performed clientside - for France, they have so many different AV services and requirements in play that limiting themselves only to clientside scanning is a no-starter.

The problem with middleboxes has nothing to do with the way they communicate with AV services. Why not reuse that interface?

The main problem with MitM proxies—even when done "properly", with a private CA root certificate installed on the endpoints—is that it defeats all the security measures built in to the clients by replacing the original server's credentials with fake credentials generated by the proxy. The MitM proxy is also in a position to tamper with the content, not just passively observe the traffic and block content that triggers the filter. This could be done better if the client and the proxy worked together (share ephemeral decryption keys with a trusted and authenticated proxy but still require end-to-end encryption and authentication) but so far no one has implemented that.

The Matrix system does not suffer from these flaws since it only shares the files themselves with the scanning service. The service can't impersonate arbitrary clients or modify file content, and it can't see anything other than the files to be scanned.